Radiation therapy is commonly used to treat solid tumors including head and neck squamous cell cancer (HNSCC); however, many patients still fail locally. Therefore, we need to find new ways to increase its effectiveness. We have been investigating inhibitors of the PI3K/mTOR pathway. In preliminary studies we found that NVP-BEZ235, a dual PI3K/mTOR inhibitor, and NVP-BKM120, a PI3K inhibitor, radiosensitize cells in vitro and induce autophagy, which we hypothesize is a cytoprotective response rather than a mode of cell death. In Aim 1 we will test this hypothesis in vitro using BKM120 and also in vivo with flank and orthotopic tumors in nude mice. We will use both genetic approaches (knocking out key autophagy genes) and pharmacologic approaches (chemicals that inhibit autophagy, Spautin1 and chloroquine). We also have preliminary data that multiple drugs that inhibit PI3K/mTOR signaling including, the 2 above and GDC-0980, GDC-0068, and RAD001, decrease O2 consumption rate (OCR) in vitro. We have also shown that BEZ235 decreases tumor hypoxia in vivo; thereby, leading us to propose a new model by which oxygenation within tumors may be modulated to increase cell killing after radiation. In Aim 2 we will investigate the mechanism(s) by which these drugs decrease OCR. We have 2 hypotheses, the first of which is that they increase Ser293 phosphorylation of the E1? subunit of pyruvate dehydrogenase (PDH), which is a critical gatekeeper of mitochondrial respiration. Phosphorylation of PDH E1? inhibits its function, hence reduces entry of pyruvate into the citric acid cycle and consequently decreases OCR. Our second hypothesis is that drugs that inhibit mTOR downregulate the expression of mitochondrial proteins that are involved in cellular respiration. In Aim 3 we will investigate whether the decrease in O2 consumption by PI3K/mTOR inhibition leads to increased radiation sensitivity in vivo. One of the ways we will do this is by using the drug GDC-0980, which does not affect intrinsic (in vitro) radiosensitivity but does reduce OCR. Hence, if this drug leads to increased radiation response in vivo, it is likely through effects on oxygenation. In Aim 3 we will also continue our screen of a 426 chemical compound library of FDA- approved agents to search for other agents that decrease OCR. We will then test the top candidates (in terms of degree of reduction of OCR) for their effects on tumor hypoxia in vivo and determine whether they have an additive effect with PI3K/mTOR inhibitors on decreasing hypoxia. Successful completion of these aims will set the stage for PI3K/mTOR inhibitors currently being tested in the clinic to be used in combination with radiotherapy for HNSCC and generate new leads for translational drugs that impact upon tumor cell oxygen metabolism.